EPROM element employing self-aligning process

ABSTRACT

A novel process is provided for fabricating contacts (46s, 40g, 46d) in a novel, completely self-aligned, planarized configuration for EPROM elements (66). The process of the invention permits higher packing densities, and allows feature distances to approach 0.5 μm and lower. The EPROM element comprises source (18) and drain (20) regions separated by a gate region (22) and is characterized by the gate region comprising two separate gates, a floating gate (40g) and a control gate (58), capacitively coupled together. The floating gate is formed on a gate oxide (38) over the substrate (16) and the gates are separated from each other and from the source and drain contacts by a dielectric (56). The EPROM element has two threshold voltages, one related to the operation of a &#34;normal&#34; MOS transistor and the other related to a &#34;programmed&#34; threshold, following programming of the transistor. Sensing the threshold voltage of the device permits a determination to be made whether the device is programmed. UV radiation erases the programming and restores and threshold voltage of the device to its pre-programmed level.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of Ser. No.07/162,822, filed Mar. 2, 1988 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to EPROM programming elements, and, moreparticularly, to a new device structure and process for preparing suchelements, resulting in a completely self-aligned structure.

2. Background of the Invention

The LOCOS (local oxidation of silicon) process for fabricating MOS(metal-oxide semiconductor) devices, especially CMOS (complementary MOS)is well-known and widely practiced throughout the semiconductorindustry. It is a suitable process for design geometries greater thanabout 1 μm feature size.

However, as feature sizes are reduced to sub-micrometer dimensions toachieve higher packing densities of devices, several problems emerge.

First, the depth of focus of the lithography stepper becomes smaller.Thus, the different heights of different features generate a depth offield problem.

Second, the spacing of contacts to the polysilicon gate and to the fieldoxide becomes critical at smaller dimensions. It will be appreciatedthat due to the use of separate alignment steps, the margin of error inaligning the contact can, if not adequate, result in either (a) etchingthrough the field oxide, with consequent shorting of the siliconsubstrate to the diffusion source or drain region by the contact, or (b)contacting the polysilicon gate, with consequent shorting of the gate tothe source or drain contact.

Such misalignments are accommodated by allowing a substantial spacebetween source, drain and gate and between source, drain and field edge.As a result, high packing density is sacrificed.

Another requirement of the present processing scheme is that the gatecontact is made to an interconnect which extends at right angle to thesource-gate-drain line. Such a contact requires considerably more areathan a contact directly down to the gate. However, the use of separatealignments dictate the present processing scheme, in order to avoidpotential misalignment problems.

Finally, a problem well-known in the art with the LOCOS process is theso-called "bird's beak" problem, which occurs where the field oxidetapers to the substrate in the source and drain regions. Such a taperresults in an electrical width smaller than the mask dimensions.

It is evident that the profusion of different heights during processingand the several alignment steps prevent efficient use of advancedlithography processes and other processes to generate high packingdensities of devices on a substrate, since the depth of field reduceswith the smaller dimensions that are needed for scaling.

The fabrication of erasable programmable read only memory (EPROM)elements is based on the LOCOS process, with its attendant problems. Itis desired to simplify the manufacturing and provide a simplerconstruction of EPROM elements.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide EPROM elementswhich are formed by a process which avoids most, if not all, theproblems associated with prior art processes.

It is another object of the invention to provide an EPROM structurewhich is efficient in its use of layout area.

In accordance with the invention, a new process is provided, which isintended to supplant the LOCOS process, for fabricating EPROM elements.The process of the invention employs planarization processes and totalself-alignment to avoid most, if not all, the disadvantages associatedwith the prior art process. As a result, high packing densities may beachieved.

Planarization avoids the depth of field problem. Self-alignment easilypermits various components of the devices to be interconnected in anymanner selected by the engineer, without the misalignment area penaltiesdiscussed earlier. The process of the invention permits the use of plugsof polysilicon, the tops of which can be contacted to reduce gatecontact area requirements. Feature distances of 0.5 μm and less may beachieved by the process of the invention.

A unique combination of masks in conjunction with a multi-layerstructure formed on the surface of a semiconductor wafer, themulti-layer structure including a buried etch-stop layer therein,defines the source, gate and drain elements and their geometry relativeto each other and to interconnects. Polysilicon plug contacts throughslots in the multi-structure layer permit vertical contact to be made tothe various elements.

The EPROM element of the invention comprises a MOS transistor comprisingsource and drain regions contacted by source and drain contacts,respectively, separated by a gate region formed on a substrate and ischaracterized by the gate region comprising two separate gates, afloating gate and a control gate, capacitively coupled together.

The floating gate is formed on a gate oxide over the substrate and thegates are separated from each other and from the source and draincontacts by a dielectric.

The process also enables efficient coupling of the floating gate to thecontrol gate by employing "peripheral overlap" of the electrodes. Inaddition, the structure enables the control gate to be efficientlydecoupled from the source and drain areas by employing a thickintervening dielectric between the areas.

The EPROM element of the invention has two threshold voltages, onerelated to the operation of a "normal" MOS transistor and the otherrelated to a "programmed" threshold, following programming of thetransistor. During normal operation, the voltage on the floating gate isabout the same as that of the drain, with the source at groundpotential. During programming, the control gate is made more positivewith respect to the drain. This control gate couples to the floatinggate, raising its potential. This in turn injects electrons into thegate oxide under the floating gate, thereby lowering the threshold ofthe device. Sensing the threshold voltage of the device permits adetermination to be made whether the device is programmed. UV radiationerases the programming and restores the threshold voltage of the deviceto its preprogrammed level.

The process of the invention requires fewer steps than the conventionalLOCOS process and provides an EPROM element having both a highercoupling between electrodes and a smaller size.

Other objects, features and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the FIGURES.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referred to in this description should be understood as notbeing drawn to scale except if specifically noted. Moreover, thedrawings are intended to illustrate only one portion of an integratedcircuit fabricated in accordance with the present invention.

FIGS. 1-13 depict the sequence of events in the process of theinvention, with the "a" designation depicting cross-sectional views andthe "b" designation depicting top plan views.

FIG. 14, on coordinates of current and voltage, is a plot of the I-Vcharacteristics of the EPROM element of the invention, as formed by theprocess steps delineated in FIGS. 1-13.

FIGS. 15a and 15b are views similar to FIG. 13a, showing the flow ofelectrons for an N-channel device during normal operation (FIG. 15a) andduring programming (FIG. 15b).

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventor for practicing the invention. Alternative embodiments are alsobriefly described as applicable.

The process of the invention, in its preferred form, is totallynon-LOCOS. However, it will be appreciated by those skilled in the artthat portions of the process may be incorporated in the present LOCOSprocess to realize the benefits disclosed herein. Further, while theprocess of the invention is directed specifically to employing siliconas the semiconductor, it will be clear to those skilled in the art thatthe teachings of the invention may be extended to other semiconductors,with suitable modifications in etch-stop layers and the like.

The process of the invention begins with the formation of an isolationtrench 10 surrounding the area in which a device, (here, an MOStransistor 14, seen in FIG. 11a), is to be fabricated in a P-well region12 in a semiconductor substrate 16. There are a plurality of suchisolation trenches 10, one associated with each device 14.

Alternatively, a conventional LOCOS oxide could be used, which wouldpermit omission of oxide layer 28a, discussed below.

As is well-known, such transistors 14 comprise source 18 and drain 20elements, with a gate area 22 therebetween to regulate the extent of aconnector 24 therebetween (seen in FIG. 10a). The formation of P-wellsis known, and hence does not form a part of this invention.

As shown in FIG. 1a, a plurality of trenches 10 surrounding well regions12 in a semiconductor substrate 16 are etched through a resist mask 23formed on top of a nitride mask 25 formed on the surface 16' of thesubstrate. The resist 23 and the nitride 25 are then stripped and thetrenches 10 are filled with an oxide 26, as shown in FIG. 2a.

The etching of the trench is done employing well-known processes, suchas an RIE (reactive ion etch) process followed by suitable wet silicondamage removal etches. The oxide 26 used to fill the trenches 10 may bea low temperature oxide or a preferred 750° C. TEOS (tetra-ethyl orthosilicate). The oxide is etched back to be planar with the surface 16' ofthe silicon substrate 16. The planarizing is done by a combination ofchemical and reactive ion etching processes commonly used in the art foroxide planarization.

The isolation trench 10 is created by etching a four-walled trench tooutline an active region of the appropriate length (X) and width (Y). Inthe process of the invention, the trench 10 has a rectangularconfiguration, when viewed in top plan view (FIG. 3b), defining anactive area (X x Y) of about 5 μm by 2 μm (for 1 μm design rules).

The isolation trench is typically about 0.5 to 1 μm wide and about 0.4to 0.6 μm deep. The size is governed by lithography alignmentconstraints and filling of the trenches by subsequently deposited oxide.

While one complete trench is shown in the FIG. 3a, it will beappreciated that a plurality of such trenches is employed on a singlesemiconductor substrate 16, as shown in FIG. 2a, each delineating anactive area of a device. Of course, as the technology of featuredefinition continues to improve with new resists and resist developmentprocesses, these dimensions will also decrease.

Next, a specific sequence of layers 28 is formed on the surface of thesubstrate 16, as shown in FIG. 3a. The particular sequence of layers 28is the crux of the invention. This multi-layer structure includes aburied stop-etch layer, essential in the practice of the invention.

First, a field oxide 28a, having a thickness of about 2,500 Å±5%, isformed on the surface of the substrate 16. The thickness of the fieldoxide 28a can be targeted consistent with the field threshold voltagerequired and subsequent implant energy available for forming N⁻connectors, described below.

The formation of the field oxide 28a in itself is not novel, and isformed by a conventional deposition process to the required thicknesses.The thickness selected depends upon the implant energy needed to implanttherethrough versus the polysilicon left to block the implant, asdiscussed below in connection with FIG. 10a.

Ideally, the field oxide 28a should be as thin as possible consistentwith the required field inversion voltages. Also importantly, thethickness of this layer 28a should be substantially uniform across thesurface of the wafer, in order to best realize the benefits of theinvention. For silicon-based devices, the field oxide comprises silicondioxide.

The field oxide layer 28a doubles as a decoupling layer between thecontrol gate electrode 58 (see FIG. 12a and the discussion associatedtherewith) and the extensions to the source and drain junctions (18' and20') formed by the connector implants (delineated 24 in FIG. 12a). Forthe most efficient programming to occur (programming is described ingreater detail near the end of this section), the floating gate 40g(shown in FIG. 9a and subsequent Figures) must be closely capacitivelycoupled to the control gate 58 and poorly coupled to the source anddrain extensions 24. The field oxide layer 28a therefore serves a dualpurpose of a conventional field oxide in the non-EPROM circuitry and asa decoupling dielectric between the control gate 56 and the source/drainareas. A typical thickness that satisfies these constraints is about2,500 Å±5%, as described above.

Next, a thin layer 28b of an etch-stop material is formed on the fieldoxide 28a. The etch-stop material 28b comprises a material having asignificantly different etch rate from silicon dioxide and is importantin the practice of the invention, as will be seen below. Forsilicon-based devices, a suitable etch-stop material comprises siliconnitride, having a thickness of about 800 Å±5%. The nitride can bedeposited by LPCVD (low pressure chemical vapor deposition) or PECVD(plasma-enhanced CVD). The minimum and maximum thickness depend on theetch uniformity of the process and apparatus.

Over the nitride layer 28b is formed another layer 28c of oxide, rangingin thickness from about 5,500 to 6,500 Å. This oxide layer isconveniently formed in the same manner as the field oxide. This oxidelayer 28c governs the depth of polysilicon plugs to be formed asdiscussed below and governs what remains of the polysilicon layer(interconnect polysilicon) following etch-back at the anti-contact mask,also discussed below, in connection with FIG. 8b. Thus, the thickness ofthe oxide layer 28c must be greater than the sum of about 4,000 Å ofpolysilicon removed and the residual interconnect thickness needed(about 2,000 Å) at that stage.

Finally, a layer 28d of polysilicon is formed on the oxide layer 28c toa thickness ranging from about 2,000 to 2,500 Å. This layer 28d has adual purpose: it serves both as an etch mask and as a lithography"enhancement" layer, as explained in connection with FIG. 5b. Whilematerials other than polysilicon may be used that have a good etch ratioto silicon dioxide, such as silicon nitride, polysilicon has severaladvantages. First, it has uniform reflectivity so that lithography iseasier to control. Second, it has a better than 20:1 etch ratio tosilicon dioxide so that it doubles as an etch mask even if overlyingresist gets eroded away during the subsequent etching, therebypreventing dimension control losses.

The foregoing layers 28a-d may be sequentially deposited in onepump-down to reduce defect density.

A layer of resist 30 is next formed on the top polysilicon layer 28d andis exposed to a "slot" mask (dashed lines 32 in FIG. 4b), whichsimultaneously provides for mutual self-alignment between source 18,gate 22 and drain 20 regions. The slot mask 32 may also includeprovision for contact to other devices via extension 33 and for contactto the gate 22 via extension 35 and any other combinations thereof. Itis a total level of interconnect, compared to conventional polysilicon,which is a half level. (The conventional half-level requires anotherlevel of interconnect to complete the connections.) In this manner,packing density of devices may be increased by nearly 50% over that ofthe prior art LOCOS process without critical alignment requirements.Further increases in density may be achieved as the technology of finerresist definition develops.

The slot mask defines the polysilicon interconnect areas in the fieldand the source/gate/drain areas of all the devices. The slot masks canbe at minimum feature and minimum feature spaces that may be definedlithographically. As can be seen, the source/gate/drain areas areself-aligned with each other so far.

The resist 30 can be exposed by electromagnetic radiation (visible, UV,X-ray, as appropriate), as is conventional, and the undesired portionsof the underlying four layers 28 are removed, such as by etching, toexpose portions of the semiconductor substrate 16 corresponding to thesource 18, gate 22 and drain 20 regions.

The four layers 28 are etched as follows: First, the exposed portions ofthe polysilicon layer 28d are etched using an RIE etcher, followed byetching the exposed portions of the oxide layer 28c, again, using an RIEetcher. This latter etch stops at the nitride layer 28b, since the etchrate of the oxide can be adjusted with etch parameters to be about fivetimes that of the nitride. This method of etching allows controlledmanufacturability of the etch, since the nitride layer 28b acts as abuilt-in "buried" etch stop.

The two etchings (of layers 28d, 28c) may be performed in one pump-down.At this point, the nitride layer 28b may be etched to the field oxidealso, if it is desired to remove nitride from underneath the polysiliconlayer 28d eventually.

The RIE etch process employs a mixture of oxygen and fluorinated gas,such as CHF₃, CF₄, NF₃, and the like. Controlled etch ratios betweenoxide and nitride are achieved by varying the ratio of the gases and, insome instances, the power of the etcher. The RIE etch process iswell-known and thus does not per se form a part of this invention.

The resist 30 is stripped and the wafer is remasked, using an "active"mask (denoted by solid lines 34 in FIG. 5b, the trench 10 being omittedfor clarity) in conjunction with a new resist layer 36. The purpose ofthis mask is two-fold. First, in the source 18/gate 22/drain 20 areas,the etch is completed to silicon or to the exposed oxide in the slots.Some of this exposed oxide in the trenches 10 will be etched (≈500 Å),corresponding to the required over-etch needed to clear the field oxideareas in the source 18, gate 22, and drain 20 openings down to thesilicon substrate 16.

In the field areas or interconnect areas over the field, the mask coversthis with resist and prevents etching. In this manner, the oxide 28a inthe field region is ultimately retained under the polysilicon extensions33, 35 in the field region, which is outside the area enclosed by theisolation trench 10 (including the trench oxide 26). This forms thepolysilicon interconnections.

The exposed portions of the nitride layer 28b are then etched, stoppingon the field oxide layer 28a, again using RIE. The etch ratio isadjusted to greater than about 3:1 nitride to oxide. The exposedportions of the field oxide layer 28a are then etched, to exposeportions of the underlying silicon 16. Again, as indicated above, theetching of the nitride layer 28b and oxide layer 28a can be donesequentially in one pump-down in the same etcher, merely changingchemistry to accommodate the nature of the respective layers.

With an oxide-to-nitride etch ratio of ≧3:1 and a non-uniformity of etchof ±10% and of the oxide layer 28c of ±10% and with the thickness of theoxide layer ranging from about 6,000 to 7,000 Å, it is possible toover-etch layer 28c and still stop on or in the nitride layer 28b. Thisgives in itself a large manufacturing advantage. In the prior artmethods of etch-back, a "timed etch" would not overcome non-uniformityproblems. A laser end-point method samples one wafer in a batch machineand is subject to the same uncertainties. Thus, the use of the "buriedetch stop" nitride layer 28b is seen to provide a unique solution to yetanother etch sequence that enhances the process of the invention. Asecond etch then removes the nitride layer 28b, stopping on the fieldoxide 28a.

With the completion of the etching down to silicon, the width W of thedevice has been defined by the trench mask 10 in FIGS. 3b and 4b as Y=W.The length of the channel (geometric), denoted L, has already beendefined in the first etch, using the slot mask 32. Thus, the source 18,gate 22 and drain 20 regions have been defined geometrically, togetherwith the device length L of the channel under the gate and the width Wunder the gate.

After etching through the field oxide layer 28a, the resist layer 36 isremoved in a conventional manner and a thin oxide film 38 (the gateoxide) is grown in the exposed portions of the semiconductor substrate16 (FIG. 6a). The gate oxide 38, as is conventional, is formed to athickness of about 150±10 Å, or less, depending on the scaling of thedevices.

A source/drain etch-out mask (denoted by solid lines 44 in FIG. 7b) isused to cover and protect the n-MOS gate areas, as shown at 42 in FIG.7a. Next, those portions of the gate oxide layer in the source 18, drain20 areas are removed such as by a wet etch dip or by RIE etching or acombination thereof. The resist is then stripped off the wafer.

A polysilicon layer is blanket-deposited to a thickness of about 7,000Å±5% (for 1 μm feature widths) and then etched or polished(chemical/mechanical) back to stop on the upper oxide layer 28c. Thepolysilicon fills all the source 18, gate 22 and drain 20 slots, as wellas all the interconnect slots 32. (The polysilicon filling the gate 22slot is denoted 40g in FIG. 7a, and in connection with EPROM structures,is referred to as the floating gate).

The polysilicon layer is polished back (chemical/mechanical), using thesame apparatus as in silicon wafer polish, to planarize the polysilicon.

Next, a doping cycle is performed. In this doping cycle, the polysilicongates 40g are not doped until the end of the process.

To this end, an oxide layer 48 is formed everywhere, for example,thermally grown at about 900° C., typically to a thickness of about 125Å±10%. This doubles as an ion implantation screen and a nitride etchstop, and accordingly, the thickness is governed by theseconsiderations. B₁₁ ions are implanted to give P⁺ doping into the N⁺areas that are open. Where it is desired to form N⁺ doping, a nitridelayer 42 about 600 to 800 Å thick (sufficient to mask POCl₃) isdeposited, masked using the N⁺ mask, and etched away together with the125 Å oxide layer 48 underneath to stop on polysilicon to expose N⁺regions, which are then counter-doped with POCl₃ (these regions werepreviously doped with boron).

The N⁺ 44 mask (shown in FIG. 7b) employs nitride as the mask for N-MOSdevices. As seen in FIG. 7a, the mask 42 protects the gate area 22against doping. This is different compared to conventional N⁺ masks,wherein the source, gate and drain are all of the same doping; that is,the NMOS areas are completely opened and the gates are not protected.

It will be recalled that the N wells 12 are already in place prior totrench formation. Of course, the source and drain polysilicon plugs ofeach isolated region or device 14 are doped to a conductivity oppositethat of the well 12 of that region, except where contacts to these areneeded, they are then the same conductivity.

The assembly is heated to about 900° C. for about 60 min., to drive thedopants to form N⁺ junctions/regions for both sources 18 and drains 20in the semiconductor. For example, as seen in FIG. 8a, doped regions18', 20' are formed by the drive. The temperature and time may bevaried, depending on the plug depth and junction depth needed.Alternatively, a rapid thermal anneal may be used to drive thejunctions.

The gate is still undoped, since doping the gate polysilicon 40g wouldresult in the dopant species penetrating the thin gate oxide 38 to theunderlying semiconductor 16, and causing potential reliability problems.

The masking nitride cap 42 is next removed over the gate areas 22.

At this stage, it is seen that the three layers (field oxide 28a,nitride 28b, and oxide 28c), are planarized, with planarized polysiliconplugs 46s, 40g, 46d to sources 18, gates 22, and drains 20,respectively, on the semiconductor substrate 16. All sources, gates,drains and interconnects are mutually self-aligned. After the definitionand drive of the plugs, a novel contact scheme, as described in detailbelow, can be implemented.

The devices now must be connected to the gate edges, as will bedescribed in detail below. The following discussion involves theformation of contacts directly over the gate electrode 40g withoutdegrading the gate oxide 38. This configuration permits scaling of thefeature sizes.

In conventional LOCOS technologies, the thickness of the polysiliconover the gate areas has to be scaled down to present reasonably planaror small step heights, as the aspect ratios get worse. A contact cannotbe made directly on this gate over the gate oxide without degrading thegate breakdown.

Thus, another aspect of the novel approach of this invention is that itallows a planar surface to be present at the contact-making step and atthe metal deposition step immediately following it, which affords amanufacturing advantage at small geometries. The inventive approach alsomakes refill technologies and methods less demanding than conventionalmethods.

An "anti-contact" resist mask (denoted by the crossed areas 50 in FIG.8b) is formed as a layer 52 on the polysilicon layers 40g, 46s, 46d andpatterned to cover the source 46s and drain 46d polysilicon and otherdesired regions of connectors. In the anti-contact mask 50, resist isleft where the contacts are required, as distinct from a conventionalcontact mask where these contact areas are normally opened.

Next, the exposed portions of the polysilicon layer 46 are etched with atimed etch to remove a specific amount of polysilicon, using an etchantthat has good selectivity to the underlying oxide is non-preferentialwith respect to N⁺ or undoped polysilicon. An example of such an etchantis chlorine-based plasma chemistry. In particular, about 4,000 Å±5% ofpolysilicon is removed. This amount will be the programming dielectriclayer plus the control gate, the formation and purpose of which arediscussed below.

The contacts can overlap the oxide because the RIE polysilicon/oxideratio evidences >20:1 selectivity. The contacts that are being definedare features in the resist and not openings in the resist as inconventional contact schemes, making lithography easier.

The resulting structure is shown in FIG. 8a. It will be seen that thepolysilicon gate 40g is recessed by the etching, since it is an areathat is not to be contacted. Other areas recessed by the etching (notshown) also will not be contacted.

All the etched back areas at this stage have a similar width in onedirection. The polysilicon in the gate is the width of the slot in whichit resides (parallel to the plane of FIG. 8a). It is thus easy to fillthese areas with spin-on glass to obtain a very flat topology as shownin FIG. 11a and as discussed in further detail below.

The patterned resist 52 is removed, leaving contact to source 46s anddrain 46d "buttons".

A connector mask (denoted in FIG. 9b by solid lines 54) is employed todefine portions 55 of the oxide layer 28c by use of a resist layer 57.These portions 55 are removed by etching, using the underlying nitridelayer 28b as an etch stop. The exposed portions of the nitride layer 28bare then removed by etching, stopping on the field oxide layer 28a.

It should be noted that this etch is identical to that discussed abovein forming the interconnects and has all the advantages referred totherein.

Implantations of appropriate ions (for N channels 24 connecting source18' and drain 20' to the gate 22' edges) are then done through theexposed portions of the field oxide 28a to connect and form thetransistors (N-type). It will be noted that the oxide to be implantedthrough is the field oxide 28a, which has been deposited to awell-controlled thickness, as described above.

The wafer is now implanted, using the N-mask, with phosphorus for then-channels (≈250 keV at 5×10¹⁴ cm⁻²). The operation is done using thesequence of N-mask and etch, N-implant, and resist strip. The source18'/drain 20' junctions are now connected to the gate region 22' (seenin FIG. 10a), completing the MOS FET device 14. The extensions 24 aredenoted "+".

Due to scattering at right angles to the implant direction, there isadvantageously an implant "tail" underneath the gate edges that resultsin a graded junction. This is beneficial for reducing hot electroneffects for short channel devices, as is well-known.

During the connector implantation, the gate polysilicon plugs 40g arealso lightly doped with the same implant. The doping is driven slightly,such as at about 800° to 1,000° C. for 15 to 30 seconds, by a rapidthermal anneal. This process activates the implants in the extensions 24but does not cause excess diffusion of the implants, thereby avoidingpenetration of the gate oxide 38. (The faster diffusion rates inpolysilicon, however, allow substantially uniform doping of the gatepolysilicon with this short cycle.) In this manner, N polysilicon plugsover the gates are formed.

A layer 56 of inter-polysilicon dielectric is next grown over theexposed areas of polysilicon (46s, 46d, 40g) and oxide 28a therebetween,to form the programming dielectric. The dielectric layer may comprise(a) an oxide, such as silicon dioxide, ranging in thickness from about100 to 200 Å, (b) an oxy-nitride about 100 Å thick formed by rapidthermal annealing of an oxide in ammonia, or (c) an oxide about 80 Åthick over which is deposited a LPCVD layer (≈800° C.) of a nitrideabout 100 Å thick, which oxide layer can be slightly oxidized (e.g., 50Å SiO₂) to give an oxide/nitride/oxide structure to seal any pinholes inthe nitride and give better dielectric integrity. The latter dielectriclayers improve the break-down voltage of the inter poly layer 56 whileimproving the coupling between the poly layers 40g and 58 (discussedbelow) because of the higher dielectric constant. Following depositionof the inter-polysilicon dielectric layer 56, the structure is as shownin FIG. 11 a. This dielectric layer 56 is used as an etch-stop in thestep described below.

A layer 58 of polysilicon is deposited everywhere and is polished backto be planar with the top of the sources 46s and drains 46d. Thedielectric layer 56 also doubles as an etch-stop over the source 46s anddrain 46d areas during the polish back. Layer 58 is termed the secondpoly level, and will comprise the control gate. A resist layer 60 isdeposited and patterned to provide openings 62 to expose portions of thepoly layer 58, as shown in FIG. 12a.

The exposed portions of polysilicon 58 are etched back to stop on thedielectric layer 56, forming openings 62. All exposed portions of thedielectric layer 56 are then etched away, the resist layer 60 isstripped, and the openings 62 are filled with oxide 64 to form an EPROMelement 66, shown in FIG. 13a.

The formation of the control gate 58 in this manner, together with theoverlap of the floating gate 40g enables coupling between the twoelectrodes to take place both in the horizontal interface between thetwo and in the vertical sidewall areas of the interfaces. Thisadditional coupling between the floating gate 40g and the control gate58, referred to as "peripheral overlap", greatly increases the couplingefficiency of the cell.

Contacts may be made to the source 46s, drain 46d, and control gate 58by means well-known in the art.

A number of modifications of the main process of the invention may bemade.

In one modification, tungsten may be used in place of polysilicon, suchas for the source/drain contacts. If tungsten is employed, then theprocess should be modified to implant the substrate first, then deposittungsten. In this modification, any polysilicon used, such as in thegates, would also be deposited after implant of the substrate, and anysubsequent implant would be to dope the polysilicon, not to form anyjunctions. The intersection of the polysilicon gates and tungsteninterconnect will be ohmic, due to the formation of tungstenpolysilicide at this intersection.

The operation of the device 66 is now described. The structure shown inFIG. 13a is an N-channel transistor with the floating gate 40g andcontrol gate 58 capacitively coupled (as distinct from electricallycoupled) through the dielectric 56.

The basic premise is that the device 66 has two threshold voltages: onerelated to the operation of a "normal" N-channel transistor and theother related to a "programmed" threshold, following programming of thetransistor.

In the programming mode, the source 46s is at 0 V (ground), the drain46d is at least 5 V and the voltage on the control gate 58 (V_(GS)) isequal to or less than that on the drain. Due to the capacitive couplingbetween the floating gate 40g and the control gate 58, the potential onthe floating gate is substantially the same as that on the control gate.The current characteristics of the device, diagrammatically shown inFIG. 14, are considered "normal" for a transistor. The electrons flowfrom the source 46s to the drain 46d, as illustrated in FIG. 15a.

If the control gate voltage is made more positive (for example, at leasttwice the drain voltage, or about 11 to 13 V), the electrons are drawninto the gate oxide 38, as illustrated in FIG. 15b. When this occurs,the trapped electrons invert the p-type silicon at the surface so thatan n-type surface layer is formed, having the characteristicsdiagrammatically shown in FIG. 14b. Consequently, the threshold voltageof the device is lowered. By sensing the threshold voltage of theelement, a determination of whether the device is programmed may bemade.

During programming, maximum efficiency is obtained if the coupling is amaximum between the floating gate 40g and the control gate 58 (obtainedby the thin dielectric 56) and minimized elsewhere between the controlgate 58 and the source/drain areas 18', 20' (obtained by the thickdeposited field oxide 28a).

The EPROM element 66 may be erased by shining UV radiation thereon.Electrons trapped in the gate oxide 38 are made to recombine with holesgenerated by the UV radiation. This restores the threshold voltage ofthe device to its pre-programmed level.

The foregoing description of the preferred embodiment of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in this art. Itis possible that the invention may be practiced in other fabricationtechnologies in MOS or bipolar or other processes. Similarly, anyprocess steps described might be interchangeable with other steps inorder to achieve the same result. The embodiment was chosen anddescribed in order to best explain the principles of the invention andits practical application, thereby enabling others skilled in the art tounderstand the invention for various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. An EPROM element (66) comprising a MOS transistorformed on a substrate (16) and comprising source (18) and drain (20)regions contacted by source (46s) and drain (46d) contacts,respectively, and a gate region (22) therebetween comprising a floatinggate (40g) and a control gate (58), said control gate formed above saidfloating gate, said floating and control gates capacitively coupledtogether and separated from each other by a first dielectric (56) andfrom said source and drain contacts by a second dielectric (64),characterized by (a) alignment of said source and drain contacts to saidfloating gate, (b) said source and drain contacts and said control gatecomprising conducting plugs, disposed in openings formed in insulatingmaterial (28) formed on said substrate, said openings defined bysubstantially perpendicular walls, said plugs terminating in upperconnecting surfaces to form a coplanar surface in an area above saidsubstrate and substantially parallel to said substrate for contacting byplanar interconnects, and (c) said source and drain plugs and saidfloating gate having a substantially identical width parallel to saidsubstrate.
 2. The EPROM element of claim 1 wherein said MOS transistorcomprises an N-channel device.
 3. The EPROM element of claim 1 whereinsaid first dielectric separating said floating and control gatescomprises a material selected from group consisting of (a) an oxide,ranging in thickness from about 100 to 200 Å; (b) an oxynitride about100 Å thick; and (c) a first layer comprising an oxide about 80 Å thickand a second layer comprising a nitride about 100 Å thick.
 4. The EPROMelement of claim 3 wherein in group (c), said second layer of siliconnitride is slightly oxidized to provide an oxide/nitride/oxidestructure.
 5. The EPROM element of claim 1 wherein said source and draincontacts consist essentially of polysilicon.
 6. The EPROM element ofclaim 1 wherein said source and drain contacts consist essentially oftungsten.
 7. The EPROM element of claim 1 wherein a portion of saidinsulating material is of sufficient thickness to capacitively decouplesaid control gate from said substrate and wherein said second dielectricis of sufficient thickness to decouple said control gate from saidsource and drain contacts.
 8. The EPROM element of claim 1 wherein saidfloating gate has a top surface and side surfaces and said control gateis capacitively coupled to said floating gate both at said top surfaceof said floating gate and around at least a portion of said surfaces ofsaid floating gate.
 9. The EPROM element of claim 8 wherein said firstdielectric is substantially uniform in thickness.